1. Field of the Invention
The invention is generally related to nuclear reactors and in particular to thermal power reactors for use in outer space.
2. General Background
Nuclear reactors designed for use in outer space may be classified according to the energy distribution of the neutrons in the core. This energy distribution can be tailored according to the amount and type of moderator (material that reduces the energy level of neutrons) and reflector (material that returns or reflects neutrons to the core region) used in and around the core. The following three classifications are generally used.
First, a fast reactor is one in which little or no moderator is used and the average neutron energy is close to that at which the fission neutrons are born. The first successful space reactor, SNAP-8, and the SP-100 currently under development are examples of this type. These are usually liquid metal cooled reactors and are characterized by relatively high specific fuel mass (Kg/Kw).
Second, an intermediate reactor is one in which the average neutron energy at which fission occurs is in the range from a few electron volts (ev) to a few thousand electron volts (Kev). An example of this is the NERVA--type propulsion reactor which is moderated partly by the graphite matrix of the fuel elements and partly by separate columns of zirconium hydride. These reactors are intended for short term operation at very high power and are relatively massive compared to more recent conceptual designs.
Third, a thermal reactor is one in which the average neutron energy at which fission occurs is less than one electron volt. At this energy level, the fission cross sections of the important fissile materials become very large and the fissile loading is reduced relative to that required in the first two reactor types. Because of the large fission cross sections thermal reactors require substantial quantities of an efficient moderator between and around the fuel elements. The relatively small amount of fissile material required in a thermal reactor provides important advantages over fast and intermediate reactors.
A number of missions in outer space are not presently feasible because of the mass of the propulsion system and/or the on-board power system. In the case of a nuclear system with a solid moderator and high power density, adequate cooling of the moderator imposes a severe mass penalty. Compliance with safety requirements also imposes additional mass penalties. A further problem limiting some missions is degradation of the reactor moderator caused by radiation damage. It can be seen from the above that a need exists for reactors used in space applications with enhanced safety, low specific mass, and capability of extended high power operation without radiation damage to the moderator.